The present invention relates to a switchgear and particularly to a switchgear operable to connect or disconnect a load circuit to an a.c. power source by use of a semiconductor switching device.
There have been proposed various switchgears of this type. One typical example of conventional capacitor switchgears operating to open or close a capacitor load circuit is formed of a thyristor switch made up of anti-parallel connected thyristors and connected between the capacitor load and the a.c. power source, and a control circuit for the thyristor switch, as disclosed for example in Japanese Patent Publication No. 34430/73. In connecting the capacitor load to the a.c. power source, the control circuit operates on the thyristor switch to turn on at a time point when the terminal voltage of the thyristor switch has become zero, so that the rush current to the capacitor is reduced. For turning off the thyristor switch, the gate firing signal to the thyristor is removed, and the thyristor switch turns off when the capacitor current reaches almost zero. However, this time point is coincident with the peak phase of the power voltage, leaving the capacitor charged to the peak power voltage. On this account, in the first cycle of power voltage following the turn-off operation, the thyristor switch is applied with this capacitor voltage plus the peak power voltage, i.e. a doubled peak power voltage. Therefore, thyristor devices used for this purpose are required to have a breakdown voltage at least twice the peak power voltage.
One method for reducing the voltage across the thyristor switch is to connect a nonlinear resistor in parallel to the thyristor switch. In this case, the nonlinear resistor needs to be selected to have a threshold characteristic approximately 2-3 times higher than the normal voltage across the tyristor switch in the off-state, or the power dissipation of the nonlinear resistor will be too large for the practical use. Accordingly, even though this voltage suppression method is employed, thyristor devices need to have a breakdown voltage two or three times the peak power voltage.
As will be appreciated from the above description, the thyristor switch used in the conventional capacitor switchgear needs to have a breakdown voltage at least twice the peak power voltage, and in consideration of the external surge voltage, thyristor devices withstanding at least three times as high as the peak power voltage must be used. This causes an increase in the number of thyristor devices in serial connection, resulting disadvantageously in a higher construction cost and also a larger power loss during the conductive period of the thyristor switch.
Such a capacitor switchgear is used for controlling the reactive power on an a.c. power line, as disclosed for example in Japanese magazine "Electric Calculation", FIG. 11, p. 258, Oct., 1969. In this case, the thyristor switch stays on while the capacitor load is connected to the power line, creating a larger power loss due to a forward voltage drop in the thyristor, that results in a low power efficiency and the need of a large and expensive cooling system. In such applications, each capacitor bank needs an individual thyristor switch, and therefore the construction cost is high and the power loss due to a forward voltage drop in the thyristor is large.
FIG. 1 is a schematic diagram of the conventional capacitor switchgear for controlling reactive power using mechanical switches.
In the figure, reference number 1 denotes an a.c. power line, 2a-2c are switches, and 3a-3c are a.c. load capacitors connected to the a.c. power line 1 through the switches 2a-2c, respectively.
Next, the operation of the above arrangement will be described. In FIG. 1, when the power-factor of the a.c. power line 1 becomes lower, the switches 2a-2c are turned on sequentially so that the capacitors 3a-3c are connected to line in accordance with the value of power-factor. In this case, a rush current 6-10 times the rates current will flow in a certain phase relationship of throwing the switches 2a-2c, resulting in a significant distortion in the a.c. power line voltage, which adversely affects other facilities (e.g., thyristor converter) connected on the same power line. On this account, the capacitors cannot have a large unit bank capacitance, and an increased number of capacitor banks are needed. This results disadvantageously in a larger installation space and a higher construction cost. In addition, when the switches are operated continually, the transient phenomena at connecting or disconnecting the capacitors impair the work life of the switches 2a-2c and capacitors 3a-3c.
To cope with these problems in the use of frequent switching operation, a capacitor switchgear is designed to connect or disconnect capacitors 3a-3c with thyristor switches 4a-4c, each made up of anti-parallel-connected thyristors as shown in FIG. 2, in place of the switches 3a-3c in FIG. 1, as disclosed for example in Japanese Patent Publication No. 40218/75.
FIG. 3 shows a basic circuit arrangement for connecting or disconnecting a capacitor 3 to an a.c. power source 1 by means of the thyristor switch 4 shown in FIG. 2.
Next, the operation of the above basic circuit arrangement will be described in connection with the waveform diagrams of FIGS. 4A, 4B and 4C. The thyristor switch 4 is off up to a time point t.sub.1, shown in FIGS. 4A and 4B and a voltage E equal to the a.c. peak power voltage shown in FIG. 4A is applied across the thyristor switch 4 as shown in FIG. 4B.
When the load capacitor 3 is to be connected to the power source, the thyristor switch 4 is controlled by a control signal provided by a control circuit (not shown) so that it is turned on in response to a zero voltage across it. Namely, the thyristor switch 4 is turned on at the time point t.sub.1 in FIGS. 4B and 4C. Then, a current shown in FIG. 4C flows through the capacitor.
To turn off the thyristor switch 4, the firing signal to the gate of the thyristors is removed, and the thyristor switch 4 becomes nonconductive at a time point t.sub.2 when the capacitor current has fallen to zero as shown in FIG. 4C. The time point t.sub.2 is coincident with the peak phase of the power voltage E, and the capacitor 3 is charged to the peak power voltage E at the time point t.sub.2.
Accordingly, when the thyristor switch 4 becomes off at t.sub.2, the capacitor 3 is left charged to the voltage E, which is added to the voltage of the a.c. power source 1, resulting in the application of a 2E voltage across the thyristor switch 4 following the time poiint t.sub.2 as shown in FIG. 4B. On this account, the thyristor devices used need to have a withstand voltage of at least 2E-3E.
During the period when the capacitor 3 is connected to the power source, the thyristor switch 4 stays on, creating a significant power loss due to the forward voltage drop of the thyristors, resulting in a lower power efficiency and the need of a large cooling system that makes the switchgear expensive.
The conventional capacitor switchgears are arranged as described above, and in the case of using a mechanical switch shown in FIG. 1, it cannot be used for a frequent switching operation and the capacitor bank cannot have a large unit capacitance due to the creation of a power voltage distortion by the rush current when the capacitors are connected. In another case of using a thyristor switch shown in FIG. 2, the forward voltage drop of each thyristor creates a large power loss and a thyristor switch is needed for each capacitor bank, resulting in a higher construction cost.